Why does lockScreen not work here?

[barryb at libero.it]

This is a similar question to that of Tiemo (why does lock screen doesn't locks the screen? (Tiemo Hollmann TB)) about finding colours under other objects (controls) but that thread evolved so fast in a single Digest I may appear a bit late. Thus the hijacked thread title.

The following handler detects the colour onto which a dragged object is dropped by moving it and getting the mouseColor but. Although it could be interpreted as a settling wobble, I would prefer not to see the little movement .
 
This works correctly, as is, but if I uncomment the lines 'lockScreen' and 'unlockScreen' then the whole handler is stopped. 

I've tried moving the commented lines around and using 'set lock screen' but the result is always the same, suggestions please.
 
-----------------------
Global gCurrBall

on Rollit
   --lockScreen
   move gCurrBall relative 20,-20  --These movements should not be seen by user-
   get the mouseColor
   put it into tColour
   move gCurrBall relative -20,20
   --unlockScreen
   if tColour is 0,0,0 then
      Dropit
   else exit to top
end Rollit

on Dropit..........
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Using Windows XPHome, Rev vs 3.5 / 4.0

Best to all 
Barry Barber